1. Field of the Invention
The present invention relates to dental apparatus and methods and more specifically to a system for orienting dental restorations to desired aesthetic reference plane(s).
2. Description of the Related Art
Dental restorations can usually be made functionally adequately in three planes of space (horizontal, vertical, frontal). However, too often dental restorations are made without sufficient attention to the aesthetic orientation of the patient's teeth. When in a proper erect standing or sitting position, the patient's maxillary anterior incisal-canine teeth line should ideally be with the horizon (horizontal reference plane) at a right angle to the patents mid-sagittal (vertical reference plane) when viewed from the patients front (frontal plane). The incisors midline should be vertical in the frontal plane (not angled). The angulations of the anterior teeth when viewed from the side of the patient (sagittal plane) is also important in diagnosing lip support for aesthetically pleasing smiles and proper facial profile. When aesthetically aligned, if the patient's head tilts from side to side, the incisal-canine teeth line will tilt with the head. To an observer, this slant of the teeth appears normal because the observer knows that when the head returns to the proper erect posture, the teeth will be horizontal again.
All three planes (horizontal, vertical, and frontal) are of aesthetic concern when diagnosing and providing dental care involving the orientation of teeth or gums, such as, restorative dentistry, prosthodontics, orthodontics, maxilla-facial surgery, periodontics, gingivoplasty and other similar procedures.
There are many reasons why a dental patient may not naturally have the ideal aesthetic reference plane. Oftentimes, the teeth or skeletal bones are not properly or symmetrically formed in either the horizontal, vertical or frontal plane. Also, incorrect head posture disorients the teeth line. That is, some people have a chronic head tilt and seldom hold their head perfectly erect due to a number of causes such as occlusion, neuro-muscular, injuries, uneven leg lengths, compensatory scoliosis or osteoporosis. Further, a patient may have an atypical or unsymmetrical lip(s), lip line(s) or laughing line(s). This can also be due a number of causes such as ptosis (drooping) of the lips on one side of a patient's smile from a neurological problem, injury, tumor or a psychological compensatory mechanism.
Traditionally, when a patient underwent dental care involving the orientation of teeth, a double-bite tray impression of the anterior teeth segment was taken and sent to a dental laboratory. The laboratory technician would then make a dental cast and mount this cast in an articulator from which he could, for example, make and inspect false teeth for the patient. When mounting the dental cast in an articulator, the technician had no information regarding the spacial positional relationship between the patient's teeth and head, and thus, no information about a proper aesthetic reference plane which would align the patient's teeth in the articulator. Without this information, the technician would routinely make the anterior teeth line parallel to the articulator frame or the table-top. This alignment would be aesthetically proper only if the patient had a natural aesthetic reference plane, which is rare. The result of not incorporating the patient's aesthetic reference plane when the patient's dental cast is mounted on an articulator is a slanted anterior incisal-canine line, slanted gingival line, or both. Moreover, when the anterior incisal-canine line is made slanted, the maxillary mid-line (mid-sagittal) is also slanted, which is one of the most aesthetically serious errors.
As aesthetics became an increasing concern in the dental field, dental care providers tend to make the incisal-canine teeth line parallel to the interpupillary eye line when the patient is looking straight ahead. With this quasi standard, if one eye of a patient was higher than the other, which often occurs, the incisal-canine line was made slanted in relation to the horizon when the patient's head is erect. Face bows and articulators were then used to aesthetically measure and align a patient's teeth so that the incisal-canine plane was parallel to the interpupillary line as a chairside procedure in the provisionals, which were then given back to the lab for correction.
Early face bows used the "Frankfort-horizontal" plane of reference, which ran from the porion (top of the auditory meatus) to the orbitale (lower border of the orbit). U.S. Pat. No. 1,052,806 discusses an early face bow which used this reference plane. When a method for locating the hinge axis was later discovered, the two posterior points of the reference plane was moved from the porion down to the transverse hinge axis of the mandible, which is usually several millimeters below the porion. However, the anterior reference point was kept at the same orbital point or level, thus creating the "axis-orbital" plane of reference. The dental industry has standardized the average axis-orbital reference plane at 22 mm below the nasion, according to research studies.
There are many aesthetic problems related to the use of the axis-orbital reference plane. A brief illustration of three such problems is provided. First, when a standardized nonadjustable face bow is properly positioned on a patient, the face bow is usually slanted upward, because the orbital reference point is at the standard location of 22 mm below the nasion. This slanted face bow, in turn, slants a bite fork vertical attachment post forward in the sagittal plane. That is, the bite fork is angled. However, when a dental cast is mounted on the articulator, the attachment post is placed in a perfectly vertical position in the sagittal plane, and the face bow is perfectly parallel to the upper frame of the articulator in the horizontal plane. This difference in the bite fork angle changes the orientation of the maxillary cast in relationship to the horizontal reference plane. This error steepens the plane of occlusion in relationship to the upper frame of the articulator and similarly prevents a patient's teeth plane from being aesthetically aligned. Second, dental casts are mounted lower in an articulator than where a patient's teeth are physically located in relationship to the patient's head. This difference gives an illusion of a retrognathic mandible, or lingually inclined (tipped in) incisors since the casts have been rotated down and back around the axis, and prevents a patient's teeth plane from being aesthetically aligned. Third, as previously discussed, a patient's head, ears and teeth are rarely perfectly parallel. When the patient's head is erect and the ears are not even, the face bow is slanted and, in turn, slants the bite fork vertical post in the frontal plane. This error slants the teeth on the articulator and similarly prevents the patient's teeth plane from being aesthetically aligned.
In addition to the above-identified shortcomings of the axis-orbital reference plane, there is growing support that an "axis-horizontal" reference plane is the better aesthetic reference plane. When this aesthetic reference plane is obtained, the incisal-canine line will be made parallel to the horizon when the patient's head is perfectly erect, regardless of the eyes or any other facial feature. This view is based on, among other things, that the human eye is very keen to perceive objects in spacial relationships to horizontal and vertical and the degree of deviation from these reference positions. For example, when someone sees a picture on a wall that is tilted, there is a natural urge to straighten the picture. Moreover, most authorities agree that the most pleasing smile line is when the labial and buccal occlusal edges of the maxillary teeth follow the curvature of the lower lip and radiate symmetrically back to the comissures of the lips. The anterior teeth should also be labial inclined (tilted out) for lip support. Since many people's head and facial features are unsymmetrical, the better method of establishing an aesthetic base line would be to have the aesthetic reference plane in the axis horizontal plane when the patient is in the proper erect position.
There is thus a need for a method and apparatus which measures a patient-specific aesthetic reference plane. There is also a need to relate this information to an articulator so that the patient's dental cast mounted therein can be aesthetically aligned in the axis-horizontal reference plane.